Project Summary/Abstract Mitochondria are membrane-rich organelles that are essential to eukaryotic life. Detailed insight has emerged into the assembly and the dynamics of mitochondrial membrane proteins, but a fundamental gap has remained in understanding mitochondrial lipids. Barth syndrome (BS) is a disorder of the mitochondrial lipid metabolism, in particular the metabolism of the mitochondria-specific phospholipid cardiolipin (CL), and thus provides a unique opportunity to address this gap in a context relevant to human health. BS is caused by mutations in tafazzin, an enzyme that catalyzes CL remodeling, i.e. the fatty acid exchange reaction by which the characteristic molecular composition of CL is created. The objective of this application is to identify mechanism and function of CL remodeling. This objective fits into our broad goals to understand the function of CL in mitochondria and to unravel the molecular pathophysiology of BS. We discovered that the global assembly of the system of oxidative phosphorylation (OXPHOS) is driving CL remodeling. We hypothesize that CL remodeling reduces the packing stress imposed on mitochondrial lipids by the extremely high protein concentration, which arises in mitochondrial membranes due to the OXPHOS system and other proteins. Thus, the function of CL remodeling is to stabilize lipid-protein interactions in order to allow the assembly of protein- crowded membranes. To test this hypothesis, we will focus on two features of BS, which are both dependent on the high concentration of membrane proteins. First, we will identify the cause of energy deficiency in BS. We will determine the effect of the complexes of OXPHOS on CL remodeling in flight muscle mitochondria and vice versa, the effect of CL remodeling on the OXPHOS system. We will then test whether disturbing either CL remodeling or the OXPHOS system has any effect on the half-life of individual components of mitochondrial membranes because our preliminary data suggest that CL remodeling is essential for CL stability. Second, we will determine the mechanism of the arrest of spermatogenesis in BS. Our preliminary data suggest that CL remodeling is essential for the formation of condensed mitochondria, a unique type of organelle that emerges during germ cell development and that accumulates a specific isoform of the ADP/ATP carrier (Ant4) at high concentration. Our data also suggest that condensed mitochondria are involved in the biogenesis of acrosomes. We will identify the role of CL remodeling in the formation of condensed mitochondria and then establish the mechanism by which condensed mitochondria support the biogenesis of acrosomes. Our application relies on cutting-edge techniques, such as lipidome-wide flux analysis with stable isotopes, cryo- electron microscopic tomography, and quantitative proteomics with isobaric labeling. The proposed study is significant because it will identify the mechanism and the function of CL remodeling, a widely conserved reaction of uncertain significance, and because it will establish the pathologic mechanism of BS.